Loss of personal possessions such as billfolds, purses, luggage and carrying cases generally, is a common occurrence through accident, inadvertence or outright theft. For example, crowded conditions at air terminals, train and bus depots, and the like, create conditions where confusingly similar luggage may be taken in error. These same conditions promote confusing situations where items of luggage are often misplaced and the attraction of certain expensive items such as cameras, sports equipment, or other personal possessions, including wearing apparel, is conducive to theft.
In recognition of the problem, known apparatus and systems have been devised to monitor the spatial relationship of an item to be protected with respect to a base station carried by an individual. A typical system is disclosed in U.S. Pat. No. 5,043,702 Kuo in which one embodiment thereof employs a radio frequency receiver disposed within an item of luggage. A corresponding transmitter is carried by the user and when the distance between the user and the luggage exceeds about ten to fifteen meters, a reduction in signal strength at the receiver is sensed which actuates an alarm and additionally electrifies a grid that is intended to deliver an electrical shock to the person carrying the luggage. Another example may be seen in U.S. Pat. No. 5,021,765 Morgan which relates to apparatus and a method for detecting the situation of a person falling overboard from a boat. An individual protected by the system carries a low frequency transmitter which is actuated when wet. To prevent spurious responses, a system of at least two detectors on the boat actuates an alarm when both the low frequency signal is transmitted and the person carrying the transmitter is outside a predetermined range of the detectors which is indicated when one detector output is substantially less than that of the other detector.
Both Kuo and Morgan are similar in that their respective disclosures rely on a reduction of received signal amplitude to indicate a spatial relationship of the item or person to be protected, as the case may be, with respect to a reference.
Such systems clearly have merit since they will perform adequately under most conditions. There are, however, situations in which the teachings of both Kuo and Morgan are inadequate. For example, a broad spectrum of electrical noise may mask the signal received by the receiver of Kuo such that regardless of the receiver distance from the transmitter carried by the individual, Kuo's receiver continues to sense a masking noise signal of substantially constant amplitude. Under these conditions the alarm would not be actuated.
A similar situation may occur in the case of the protective system disclosed by Morgan in that a strong interference signal may mask the output of the low frequency transmitter carried by an individual such that the two or more detectors may not sense any variation in signal level should the protected individual move out of a predetermined range from the detectors.
It becomes apparent, therefore, that any system relying on the detection of reduced signal strength of a received low level transmitted signal is subject to deception imposed by strong, broad band interference or noise signals; both are expected to be prevalent in those environments where a protection system is often most needed.